Polystyrene is a widely used commercially available plastic. Pure polystyrene is a hard plastic having limited flexibility and is thus well suited to applications where its level of stiffness is acceptable. In other situations, the limited flexibility or rigidity of pure polystyrene may be unacceptable. In these situations, styrene-based copolymers, and particularly polystyrene resins that are modified with organic rubber particles or other elastomers, have been found useful. These rubber modified styrene compositions are typically referred to as high impact polystyrene, or HIPS.
In the final HIPS product, the rubber particles are incorporated into the HIPS product, wherein the incorporated rubber particles are crosslinked and the level of crosslinking is determined by the manufacturing process and can vary considerably. In certain applications it may be desirable to achieve a HIPS product having a low level of crosslinking.
Swell index values are a direct measure of the state of cross-linking in the rubber. As the level of cross-linking increases, swell index values decrease. It has been observed that a higher swell index may confer some desirable properties to HIPS. It would therefore be desirable to produce a HIPS product having an increased swell index in order to reduce crosslinking and thus have a final HIPS product having desirable properties.